Nucleophilic substitution: Halogenoalkanes contain polar bonds.

The idea

Halogens are more electronegative than carbon, so the shared pair in a carbon–halogen bond is pulled towards the halogen. The bond is therefore polar: the carbon carries a small positive charge, δ⁺, and the halogen a small negative charge, δ⁻. That electron-deficient carbon is the site every reaction of a halogenoalkane starts from, because a nucleophile is attracted to positive charge. Polarity falls from C–F through C–Cl and C–Br to C–I as electronegativity falls down the group, so C–I is the least polar bond of the four. Do not turn that into a prediction about rate: iodoalkanes still react fastest because the C–I bond is the weakest, and bond enthalpy, not polarity, controls how quickly the bond breaks.

What earns the marks

• Stating that the halogen is more electronegative than carbon.
• The δ⁺ label on the carbon and the δ⁻ label on the halogen, shown on the bond.
• Identifying the δ⁺ carbon as the atom attacked by a nucleophile.
• The correct order of bond polarity, C–F most polar through to C–I least polar.

Common mistakes and how to put them right

• Labelling the carbon δ⁻ and the halogen δ⁺, which reverses the whole mechanism.

• Writing full charges C⁺ and Cl⁻ rather than the partial charges δ⁺ and δ⁻.

• Claiming C–I is the most polar bond because iodine is the largest atom.

• Predicting that the most polar bond reacts fastest, when the rate follows bond enthalpy instead.

Exam tips

• Always draw the δ⁺ and δ⁻ on the bond itself, not floating next to the molecule.
• Say "more electronegative than carbon" explicitly; naming the halogen as electronegative on its own rarely scores.
• Keep polarity and bond enthalpy as two separate arguments so you do not use one where the other is wanted.